The present invention relates to limiting the amplitude of a transmission signal, e.g., a telecommunication signal to be transmitted via a radio station.
In telecommunications systems, usually a large number of communication channels is transmitted together via the same transmission medium, e.g., a radio frequency band. Various access schemes for placing communication channels on the transmission medium are known. A well-known scheme is CDMA (Code Division Multiple Access) where a number of different communication channels is transmitted simultaneously in a radio frequency band in such a way that they overlap in both the time domain and the frequency domain.
In order to distinguish each communication channel signal from the other communication channel signals, each communication channel signal is encoded with one or more unique spreading codes, as is well-known in the art. By modulating each of the communication channel signals with a spreading code, the sampling rate (i.e., the xe2x80x9cchip ratexe2x80x9d) may be substantially increased in accordance with a spreading factor. For example, each communication channel signal is modulated in accordance with a digital modulation scheme, e.g., a quadrature amplitude modulation (QAM) or a phase shift keying (PSK) technique. Consequently, an in-phase and quadrature component signal is produced for each communication channel signal. QAM and PSK are well known in the art. The in-phase and quadrature component signals associated with each of the communication channels are then encoded using a unique spreading code sequence. The resulting in-phase and quadrature component signal pairs are sampled (i.e., at the chip rate) and individually weighted. The in-phase and quadrature component signals are eventually combined to form a composite in-phase signal and a composite quadrature signal. The composite in-phase signal and the composite quadrature signal are then separately filtered by a low-pass, pulse shaping filter. Subsequent to filtering, the composite in-phase signal and the composite quadrature signal are modulated by a cosine-carrier and a sine-carrier respectively and combined into a single, multicode transmission signal, e.g., a CDMA signal. The single, multicode transmission signal is then upconverted by a carrier frequency and the signal power associated with the transmission signal is boosted by a high power amplifier prior to transmission. At the receiving unit, the baseband signal associated with each of the communication channel signals is extracted from the transmission signal by demodulating and decoding the transmission signal using the carrier frequency and the various spreading codes. Furthermore, it will be understood that in a typical cellular telecommunications system, the transmission source may, for example, be a high power base station, and the receiving entity may, for example, be a mobile station (i.e., a mobile telephone).
When there is an especially large number of communication channel signals, it is sometimes preferable to generate two or more transmission or carrier signals, wherein each of the two or more carrier signals is modulated with its own unique carrier frequency. The two or more modulated carrier signals are then independently amplified by a corresponding high power amplifier prior to transmission, or alternatively, the two or more modulated carrier signals are combined into a single, complex transmission signal, which is then amplified by a single, high power amplifier prior to transmission.
As one skilled in the art will readily appreciate, CDMA substantially increases system bandwidth, which in turn, increases the network""s traffic handling capacity a whole. In addition, combining independent carrier signals into a single complex transmission signal, as described above, is advantageous in that a single high power amplifier is required rather than a separate high power amplifier for each independent carrier signal. This is advantageous because high power amplifiers are expensive, and employing one high power amplifier in place of many will result in a substantial cost savings.
Despite the advantages associated with CDMA, combining multiple communication channel signals and/or independent carrier signals, in general, significantly increases the peak-to-average power ratio associated with the resulting transmission signal. More specifically, the peak-to-average power ratio for a transmission signal can be determined in accordance with the following relationship:
PRPTA=PRF+10*log (N)
wherein PRPTA represents the peak-to-average power ratio of the corresponding composite signal, PRF represents the power ratio of the low pass, pulse shaping filter and N represents the number of communication channels which make up the carrier (CDMA) signal.
The problem associated with large peak-to-average power ratio is that it diminishes the efficiency of the high power amplifier in the transmitter. Efficiency as one skilled in the art will readily understand, is measured in terms of the amount of output power (i.e., Pmean) divided by the amount of input power (i.e., Pdc+Ppeak). As Ppeak (i.e., peak power) increases relative to Pmean, the efficiency of the high power amplifier decreases.
One possible solution is to simply limit or clip the amplitude (i.e, Ppeak) of the carrier signal. Unfortunately, this is likely to result in the generation of intermodulation products and/or spectral distortions. Intermodulation products and/or spectral distortions are, in turn, likely to cause interference between the various communication channel signals. Accordingly, this is not a preferred solution.
Another possible solution is to design a more complex high power amplifier, one that can tolerate and more efficiently amplify (CDMA) carrier signals that exhibit large peak-to-average ratios. However, this too is not a preferred solution as the cost of high power amplifiers are generally proportional to complexity. Accordingly, this solution would result in driving up the cost of the telecommunications device that houses the high power amplifier.
U.S Pat. No. 5,621,762 (xe2x80x9cMiller et al.xe2x80x9d) offers yet another possible solution for the peak-to-average power ratio problem, that is to limit the peak-to-average power ratio before the soon-to-be transmitted telecommunications signal is filtered and subsequently amplified. More specifically, Miller describes a peak power suppression device for reducing the peak-to-average power ratio of a single code sequence at the input of the high power amplifier. The peak power suppression device employs a digital signal processor (DSP) which receives the single code sequence, maps the code sequence onto a symbol constellation diagram, predicts an expected response from the pulse shaping filter and limits the amplitudes appearing on the symbol constellation diagram in accordance with the expected response of the pulse shaping filter.
The primary problem with the solution offered in Miller is that the peak power suppression device is incapable of coping with the high data bit rates encountered in telecommunications systems such as CDMA. Further, the device is incapable of coping with multiple carrier channel signals and/or multi-code sequences. For example, the peak power suppression device described in Miller is inherently slow, as evidenced by the fact that it employs a DSP (Digital Signal Processor), and by the fact that the DSP has the time necessary to execute a pulse shaping filter prediction algorithm. Therefore, a need exists for a telecommunications signal amplitude limitation device that is capable of limiting the peak-to-average power ratio of a telecommunications signal before it is filtered and subsequently amplified, and additionally, is capable of handling significantly higher bit rates, multiple code sequences, and multiple CDMA carrier signals.
It is therefore object of the invention to provide a method and apparatus for limiting the amplitude of a complex transmission signal comprising a plurality of carrier signal having high data rates.
This object of the invention is solved by an apparatus limiting an amplitude of a transmission signal, comprising: estimation means for estimating the amplitudes of each of a plurality of complex digital carrier signals based on their complex signal components, each of the signals including digitally encoded information transmitted via at least one communication channel; determining means for calculating a maximum amplitude based on the plurality of estimated amplitudes and for determining at least one amplitude scaling factor based on the maximum amplitude; scaling means for scaling the complex components of each of the plurality of complex digital carrier signals based on the at least one amplitude scaling factor; and combining means for combining the amplitude limited complex carrier signals to form the transmission signal.
According to the invention, the amplitudes of each of a plurality of complex digital carrier signals is estimated based on their complex signal components. The computed amplitudes are then used to determine at least one scaling factor for scaling the complex components of each of the plurality of complex digital carrier signals prior to combining the complex amplitude limited carrier signals to form the transmission signal.
Limiting the amplitude of each of the plurality of carrier signals allows to efficiently reduce the maximum amplitude of the complex transmission signal, thus eliminating the need for multiple power amplifiers or a single large power amplifier. Further, this allows to combine an arbitrary number of carrier signals and to process complex digital carrier signals having very high frequencies, e.g., in CDMA telecommunications applications.
Advantageously, the amplitudes of the individual carrier signals may be iteratively estimated using the CORDIC algorithm. The amplitude of a signal may be estimated with a sufficient accuracy employing at least two iterations according to the CORDIC algorithm.
In order to further reduce the computation effort, the number of bits used for a representation of the complex signal components may be reduced and absolute values of the complex components of the carrier signals prior to estimating the amplitudes may be determined. Further, the number of bits used for a digital representation of the estimated amplitudes may advantageously be reduced, at still sufficient accuracy, in order to still further reduce computation requirements.
The at least one amplitude scaling factor may also be a function of a clipping amplitude of an amplifier and the clipping amplitude may be a function of a pulse shaping filter.
Further, the at least one scaling factor may be computed as the largest integer smaller than the logarithm dualis of the maximum amplitude divided by the clipping amplitude.
The object of the invention is further solved by an apparatus for limiting an amplitude of a transmission signal, comprising: estimation means for estimating the amplitudes of each of a plurality of complex digital carrier signals based on their complex signal components, each of the signals including digitally encoded information transmitted via at least one communication channel; determining means for calculating a maximum amplitude based on the plurality of estimated amplitudes, including a first look up table for determining a first amplitude scaling factor based on the maximum amplitude and a second look up table for determining a second scaling factor; scaling means for scaling, in a coarse clipping operation, the digitally represented complex components of each of the plurality of complex digital carrier signals by deleting a number of low significance bits of the digital representations of the components, the number being determined by the first amplitude scaling factor, and for scaling, in a fine clipping operation executed after the coarse clipping operation, by multiplying the digital representations of each of the complex components with the second amplitude scaling factor; and combining means for combining the amplitude limited complex carrier signals to form the transmission signal.
In order to allow a shift scaling of the complex components of the carrier signals, a first look-up table may be used for determining at least one shift factor based on the maximum amplitude. Further, in a course clipping operation the digitally represented complex components of each of the plurality of complex digital carrier signals may be scaled by deleting an number of low significance bits of the digital representations of the components, the number being determined by the at least one shift factor.
This deleting of low significance bits may efficiently be executed by shifting the digital representations of the complex components in a register by a number of register locations determined by the at least one shift factor.
To increase accuracy of the scaling operation, a second look-up table may be provided for determining a second scaling factor, to be used in a fine clipping operation executed after the coarse clipping operation for multiplying the digital representations of each of the complex components with the second scaling factor.
Further, the object of the invention is solved by a method for limiting an amplitude of a transmission signal, comprising the steps of: estimating the amplitudes of each of a plurality of complex digital carrier signals based on their complex signal components, each of the signals comprising digitally encoded information transmitted via at least one communication channel; calculating a maximum amplitude based on the plurality of estimated amplitudes; determining at least one amplitude scaling factor based on the maximum amplitude; scaling the complex components of each of the plurality of complex digital carrier signals based on the at least one amplitude scaling factor; and combining the amplitude limited complex carrier signals to form the transmission signal.
Still further, the object of the invention is solved by a method for limiting an amplitude of a transmission signal, comprising the steps of: estimating the amplitudes of each of a plurality of complex digital carrier signals based on their complex signal components, each of the signals comprising digitally encoded information transmitted via at least one communication channel; calculating a maximum amplitude based on the plurality of estimated amplitudes; determining a first amplitude scaling factor based on the maximum amplitude using a first look up table and determining a second amplitude scaling factor using a second look up table; scaling, in a coarse clipping operation, the digitally represented complex components of each of the plurality of complex digital carrier signals by deleting a number of low significance bits of the digital representations of the components, the number being determined by the first amplitude scaling factor, and scaling, in a fine clipping step executed after the coarse clipping step, by multiplying the digital representations of each of the complex components with the second amplitude scaling factor; and combining the amplitude limited complex carrier signals to form the transmission signal.
Further advantageous embodiments of the invention are disclosed in further dependent claims.